Thermal compensation for burst-mode laser wavelength drift

ABSTRACT

An apparatus comprising a laser comprising an active layer and configured to emit an optical signal, wherein a temperature change of the laser causes the optical signal to shift in wavelength, and a heater thermally coupled to the active layer and configured to reduce a wavelength shift of the optical signal by applying heat to the active layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 14/969,610, filed on Dec. 15, 2015 by FutureweiTechnologies, Inc. and titled “Thermal Compensation for Burst-Mode LaserWavelength Drift,” which is a continuation-in-part of U.S. patentapplication Ser. No. 14/509,662, filed on Oct. 8, 2014 by FutureweiTechnologies, Inc. and titled “Thermal Compensation for Burst-Mode LaserWavelength Drift,” now U.S. Pat. No. 9,246,307, all of which areincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Passive optical networks (PONs) have user or customer end devices andoperator end devices in communication with each other. PONs may employtime-division multiplexing, in which end users share a wavelength indifferent time periods to communicate with the operator end (e.g.,optical line termination (OLT)) via an upstream link. Accordingly, sometransmitters on the user or customer side, such as those in an opticalnetwork unit (ONU), may have lasers working in a burst mode. In theburst mode, an ONU transmitter may be assigned a small time period andmay send upstream signals only within its own time period. At othertimes, the ONU transmitter may have a bias current (or voltage) belowits threshold current value (e.g., zero bias current), and thereforestay inactive.

When a burst-mode laser is enabled, it may emit or transmit an opticalsignal, on which radio frequency (RF) signals may be added. Duringemission, a temperature of the laser chip may increase slowly, causingthe optical wavelength to drift or shift. In a time- andwavelength-division multiplexing (TWDM)-PON system that shares both timeand wavelengths, multiple wavelengths may be used in both a downstreamdirection and an upstream direction. In the upstream direction, forexample, a demultiplexer (DeMUX) may be used to separate differentwavelengths sent from multiple ONUs. Each output channel in the DeMUX,similar to a filter, may have pass bands of various shapes such as aflat shape or a Gaussian shape. The wavelength shift of an opticalsignal during a burst period may cause problems at the filter. Forexample, if the peak-intensity wavelength of the optical signal is closeto an edge of the filter pass band, after wavelength shift a portion ofthe optical signal may be filtered out because the shifted wavelengthfalls out of the pass band. The optical signal may consequently vary inpower, which may cause data error problems. Therefore, the wavelengthshift of burst-mode lasers is a problem to be solved.

SUMMARY

In an embodiment, the disclosure includes an apparatus comprising aburst-mode laser comprising an active layer and configured to emit anoptical signal during a burst period, wherein a temperature change ofthe burst-mode laser causes the optical signal to shift in wavelength,and a heater thermally coupled to the active layer and configured toreduce a wavelength shift of the optical signal during the burst periodby applying heat to the active layer based on timing of the burstperiod.

In another embodiment, the disclosure includes a method for temperaturecompensation during operation of a burst-mode laser that is thermallycoupled to a heater, the method comprising receiving a burst enablesignal indicating the start of a burst period, emitting an opticalsignal with at least one wavelength during the burst period, andsubstantially maintaining a temperature of the burst-mode laserthroughout the emission of the optical signal to reduce wavelength shiftof the optical signal, wherein substantially maintaining the temperaturecomprises applying, using the heater, heat to the burst-mode laser basedon the burst enable signal.

In yet another embodiment, the disclosure includes a laser systemcomprising a burst-mode laser comprising a metallic layer that serves asan electrode pad for the burst-mode laser, and an electric heatersituated atop the burst-mode laser and comprising a first titanium (Ti)layer atop the metallic layer, a silicon dioxide (SiO₂) layer atop thefirst titanium layer, a second Ti layer atop the silicon dioxide layer,and a platinum (Pt) layer atop the second titanium layer, wherein thesecond Ti layer and the Pt layer serve as a heating pad for the electricheater, and wherein the SiO₂ layer has a thickness no more than 300nanometers to allow efficient heat transfer from the electric heater tothe burst-mode laser, and to block current injection from the heatingpad to the electrode pad.

These and other features will be more clearly understood from thefollowing detailed description taken in conjunction with theaccompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following brief description, taken in connection with theaccompanying drawings and detailed description, wherein like referencenumerals represent like parts.

FIG. 1A and FIG. 1B are exemplary simulation results of temperatureincrease and wavelength shift, respectively, for a distributed feedback(DFB) laser within one burst time period without temperaturecompensation by a heater.

FIG. 2A shows an exemplary profile of a heating current used in asimulation.

FIG. 2B shows an exemplary profile of a laser enable voltage used in thesame simulation as FIG. 2A.

FIG. 3 shows exemplary simulation results of temperature changes of aburst-mode laser under three different conditions: (1) heater only, (2)laser only, and (3) both heater and laser turned on.

FIG. 4A shows another exemplary profile of a heating current.

FIG. 4B shows an exemplary profile of a laser enable voltage used in thesame simulation as FIG. 4A.

FIG. 5 shows another exemplary simulation results of temperature changesof the burst-mode laser under three different conditions: (1) heateronly, (2) laser only, and (3) both heater and laser turned on.

FIG. 6 shows a perspective view of an embodiment of a laser system.

FIG. 7A shows a cross-sectional view of the laser system of FIG. 6 alongthe A-A line.

FIG. 7B shows a perspective cross-sectional view of the laser system ofFIG. 6 along the B-B line.

FIG. 7C shows a perspective cross-sectional view of a buriedheterostructure DFB laser.

FIGS. 8A-8D show embodiments of four driving circuits, each forgenerating a heating current that feeds into a heater.

FIG. 9 is a flowchart of an embodiment of a method for temperaturecompensation during operation of a burst-mode laser.

FIG. 10 is a schematic diagram of an embodiment of a PON.

DETAILED DESCRIPTION

It should be understood at the outset that, although an illustrativeimplementation of one or more example embodiments are provided below,the disclosed systems and/or methods may be implemented using any numberof techniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents. The drawing figures are not necessarily to scale.Certain features of embodiments may be shown exaggerated in scale or insomewhat schematic form and some details of conventional elements maynot be shown in the interest of clarity and conciseness.

The present disclosure teaches example embodiments to reduce thewavelength shift of a burst-mode laser in a burst period by stabilizinga temperature of the laser during emission of optical signals. In anembodiment, an electric heater is placed close to an active layer of thelaser to heat the laser anode surface. Heat may be applied to the laserbefore a burst period or right at the start of the burst period torapidly raise the temperature of the active area. Then, as the laserstarts emitting an optical signal shortly after the start of the burstperiod, the amount of applied heat may be reduced, causing lasertemperature to fall slowly over a short period, e.g., about 10microseconds (μs). During a latter portion of the burst period, the heatmay be turned off completely to further decrease temperature. In effect,the temperature rise caused by emitting the optical signal is balancedor compensated by the temperature decrease caused by the reduction ofheat applied on the laser. Accordingly, the overall temperature of thelaser may be stabilized, which in turn reduces wavelength shift in theburst period. The disclosed embodiments may lower upstream wavelengthdrift and keep receiver input optical power almost constant, therebyimproving the performance and quality of a TWDM-PON system. A Gaussianshaped pass-band DeMUX may be used, in essence allowing more choice inthe design of TWDM-PON system components.

An optical signal is defined herein as at least one optical wave havingat least one optical wavelength and carrying any type of signal (e.g.,RF signal). When an optical wave without any RF signal is emitted, e.g.,for reference purposes, the wavelength information and/or power of theoptical wave may still be considered types of signals. Embodimentsdisclosed herein may be applied to any suitable type of burst-modelasers including DFB lasers and distributed Bragg reflector (DBR)lasers. Further, a burst-mode laser disclosed herein may be locatedanywhere and be used in any suitable system. For example, the burst-modelaser may reside in a transmitter at the user end (e.g., in an ONU) orat the operator end (e.g., in an OLT) of a PON system. In the presentdisclosure, functioning of the heat compensation may be discussed first,followed by the structures and fabrication details of the heater.

When an injection current makes a laser emit an optical signal, thecurrent also heats the laser because the laser has an ohmic contactresistance. The heating causes a temperature of the laser to increase.The temperature of some burst-mode laser chips may be regulated viathermal electric control (TEC), in which case a stable temperaturegradient may be formed after several hundred microseconds. Nevertheless,the laser chip temperature may still change due to the applied currentand the optical signal.

FIG. 1A and FIG. 1B are exemplary simulation results of temperatureincrease and wavelength shift, respectively, for a DFB laser within aburst time period of 125 μs without thermal compensation by a heater.Specifically, FIG. 1A shows the temperature increase with time in anactive area of a DFB laser during the burst period with the x-axis beingtime and the y-axis being the laser active region temperature. FIG. 1Bshows the wavelength increase (i.e., red drift) of an optical signalemitted by the DFB laser during the burst period with the x-axis beingtime and the y-axis being the amount of laser wavelength drift. Anaverage temperature increase for laser optical confinement area may beabout 1 Kelvin (K) to 1.5 K over a burst period of about 125 μs, wherethe temperature increase may also depend on an average output opticalpower of the laser. FIG. 1A further shows that, at the beginning ofburst period, the temperature change is relatively faster, and that,toward the later part of the burst mode period, temperature change mayenter into a near-saturation region and vary at a slower slope. Thelaser temperature change results in an optical wavelength shift. Asshown in FIG. 1B, around the working ambient temperature of about 30degrees Celsius (° C.) (or 303 degrees K), 1 K in laser temperatureincrease may cause the optical wavelength to shift about 0.105 nanometer(nm). Accordingly, an empirical formula may be concluded from thesimulation:dλ=0.105 nm/k*dT,  (1)where dλ denotes wavelength shift and dT denotes temperature change.However, these values are only intended as examples, as temperatureincrease and wavelength shift may also depend on more factors such as anoutput power level of the laser.

In order to reduce the wavelength drift when a laser operates in a burstmode and thus stabilize the wavelength, an active area temperature ofthe laser may be stabilized to reduce temperature change. In anembodiment, a heater may be fabricated on the top of an anode electrodepad for the burst-mode laser. Therefore, the heater resides right on topof the laser active region so that it may heat the laser active areaquickly and efficiently. For example, as shown in FIG. 3 (furtherdescribed below), with application of heat the laser temperature mayrise 1° C. within about 1 μs.

Various embodiments of heat application dynamically based on a burstenable signal are disclosed herein. In a first embodiment, a burstenable signal (e.g., an ONU transmitter enable signal) and a heatingcurrent are applied simultaneously. In other words, the heater startsheating when the laser enabling signal turns on. The burst enable signalmay be implemented as any suitable signal (e.g., voltage or current).The burst enable signal controls timing of a burst period and indicatesthe start of the burst period. The burst-mode laser may start emittingan optical signal shortly (e.g., one μs) after the burst enable signalturns from a logic low to a logic high. A heating current for the heatermay be designed to have any suitable profile. FIG. 2A shows an exemplaryprofile of a heating current used in a simulation, and FIG. 2B shows anexemplary profile of a laser enable voltage used in the same simulation.

As shown in FIG. 2A, the heating current profile has three temporalportions associated with three different steps. The first step is from astart of the burst period to about 1 μs, where the current is set to amaximal level (e.g., about 56 milliamperes (mA)). The second step isfrom about 1 μs to about 10 μs, where the current is reduced to anintermediate or lower level (e.g., about 18.8 mA). The third and laststep is from about 10 μs to the end of the burst period, where theheater is turned off with zero current. As shown in FIG. 2B, the burstenable signal stays high throughout the burst period, and turns low atabout 125 μs to end the burst period. The laser may start emitting theoptical signal with at least one wavelength shortly (e.g., 1 μs) afterthe burst enable signal turns high (the laser warms up).

FIG. 3 shows exemplary simulation results of temperature changes of aburst-mode laser under three different conditions. Condition (1),represented by square markers, has the heater turned on with a currentprofile shown in FIG. 2A, and has the laser turned off. Condition (2),represented by triangle markers, has the laser turned on with a burstenable voltage profile shown in FIG. 2B, and has the heater turned off.Condition (3), represented by diamond markers, has both the heater andthe laser turned on according to the profile in FIG. 2A and FIG. 2B,respectively. As shown in FIG. 2A, the heating current is first set to amaximal level for a short period (e.g., about 1 μs). As shown forcondition (1) of FIG. 3, the maximal heating current heats the laseractive area quickly, causing an average temperature to rise for aboutone degree Celsius (e.g., from about 303 K to about 304.1 K) in thefirst microsecond. After the first microsecond, the heating current maydecrease to stop the laser temperature from continuing to increase. Asshown in FIG. 2A, the heating current may be lowered (e.g., from about56 mA to about 18.8 mA) for another short period (e.g., about 10 μs). Asshown for condition (1) of FIG. 3, the lower current causes the laserchip temperature to vary or fluctuate over this period (e.g., betweenabout 303.8 K and about 304.2 K). After about 10 μs, the heating currentis turned off, and the laser temperature may naturally decrease over therest of the burst period (e.g., from about 303.9 K to about 303.1 K).

Condition (2) of FIG. 3 is similar to FIG. 1B, where the lasertemperature increases as a result of its bias and modulation current.Combining conditions (1) and (2) for condition (3) in FIG. 3, it isclear that the decreasing temperature of the heater can compensate forthe increasing temperature of the laser, resulting in a largelystabilized temperature profile. Under condition (3), the heating currentand the laser enable signal are turned on simultaneously, causing thelaser temperature to stay between about 303.9 K and about 304.2 Kthroughout the burst period. As temperature variation is reduced fromabout 1 K to less than about 0.3 K, wavelength shift is reducedaccordingly. Applying equation (1), dλ=0.105 nm*dT, and assuming atemperature change dT of 0.3 K, the wavelength shift would besubstantially stable with a shifting range within about 0.0315 nm. Thecorresponding frequency shift would be within the range of 5 gigahertz(GHz). More generally, when a duration of the burst period is about 125microseconds, the temperature of the burst-mode laser may be controlledto change no more than about ±0.2° C., and the wavelength shift of theoptical signal may be no more than about ±0.02 nm.

In a second embodiment of applying heat to a laser based on a burstenable signal, the heat may be applied prior to the start of the burstperiod. FIG. 4A shows an exemplary profile of a heating current used ina simulation of the second embodiment, FIG. 4B shows an exemplaryprofile of a laser enable voltage used in the same simulation, and FIG.5 shows exemplary simulation results of temperature changes of theburst-mode laser under three different conditions. As one of ordinaryskill in the art would understand similarities between FIG. 4A and FIG.2A, between FIG. 4B and FIG. 2B, and between FIG. 5 and FIG. 3, in theinterest of conciseness, further discussions focus on differences amongthem. In the second embodiment, the burst-mode laser is preheated by theheater at a maximal level heating current to cause its temperature torise about 1 K before the start of the burst period. As shown in FIG.4A, a maximal current of about 30 mA is supplied to the heater for afirst period (e.g., about 5 μs) before the laser burst enable signal isturned on. Compared with FIG. 2A, where the maximal current was about 56mA for the same laser, the maximal current may not need to be as highbecause the duration of heating is now longer. When the burst enablesignal is turned on, the heating current is reduced to a lower value tosubstantially maintain its temperature or have its temperature decreasevery slowly for about 10 to 15 μs. After that, the heater is turned offfor the rest of the burst period, causing the laser temperature to drop.As discussed above, the laser temperature decrease caused by theturned-off heater compensates for the laser temperature rise caused bybias and modulation current. As a result, when both the laser and theheater are turned on as specified in FIG. 4A and FIG. 4B, the overalltemperature is substantially stabilized, varying between about 303.9 Kand about 304.2 K throughout the burst period, as shown in FIG. 5.

When a heating current has a simple step profile, such as shown in FIG.2A and FIG. 4A, a secondary peak temperature may be observed. Forexample, the laser temperature reaches a peak of about 304.2 K at about10 μs in FIG. 3 and reaches a small peak of about 304.05 K at about 20μs in FIG. 5. In a third embodiment, the heating current profile maytherefore be altered to reduce or avoid the secondary peaks. In anembodiment, the heating current may be set to quickly reach a maximallevel, and then decrease continuously over a time period. For example,an exponential continuous decrease may be realized using aresistor-capacitor (RC) circuit. The current may last until the end ofthe burst period or be turned off in the middle of the burst period,depending on factors such as the maximal current level and rate ofcurrent decrease. By properly designing the heating current profile, theoverall temperature of the laser may be even more stabilized, furtherreducing the wavelength shift of optical signals.

FIG. 6 illustrates a perspective view of an embodiment of a laser system600, which may comprise a burst-mode laser 610 and an electric heater620. The laser system 600 may be implemented using a single chip (e.g.,all components may be monolithically fabricated on the same chip) ormultiple chips (e.g., some components may be fabricated separately andthen bonded together). Depending on the design, the heater 620 may beconsidered part of the laser 610 or thermally coupled to the laser 610.For example, when all components of the laser system 600 aremonolithically fabricated on the same chip, the laser system 600 maysometimes simply be referred to as a laser. As shown in FIG. 6, theheater 620 with bonding pads may be integrated atop a laser anodeelectrode pad 612 for the laser 610 such that the heater 620 is locatedright on top of an active layer or region of the laser 610. Since theheater is close to the active layer, when a heating current runs throughthe heater 620 via heating pads, the heater 620 may increase atemperature of the laser active layer quickly (e.g., an increase ofabout 1 K within about 1 μs). In an embodiment, the heater may be madeof titanium (Ti) and platinum (Pt) thin films or layers, where Ti isdeposited atop a silicon dioxide (SiO₂) layer that is used to isolatelaser injection and heating current. The Pt layer may serve as the mainheating layer. The bonding pads have a gold (Au) layer atop the Ptlayer. The gold layer provides for wire bonding. In an embodiment, thebonding pads portion may have the gold layer while the rest of theheater 620 and its corresponding Pt layer do not have the gold layer.Underneath the isolating SiO₂ layer, the laser anode electrode maycomprise Ti, Pt, and/or gold layers. It should be understood thatdirectional terms mentioned herein, such as top, bottom, atop, on topof, below, underneath, vertical, horizontal, etc., are meant to berelative and do not impose any limitation on the orientation of thelaser system or its components.

FIG. 7A shows a cross-sectional view of the laser system 600 of FIG. 6along the A-A line, in essence presenting a detailed vertical layout ofthe laser system 600. The laser system 600 may be fabricated using anysuitable technology in a layer-by-layer fashion. Layers of the lasercomprise, from bottom to top, a substrate 710 made of n-type indiumphosphide (n-InP), an active layer 720, a cladding layer 730 made ofp-type InP, an ohmic contact layer 740 made of heavily-doped indiumgallium arsenide (p⁺-InGaAs), and an electrode layer 750 made of metalssuch as Ti/Pt/Au layers. The electrode layer 750 may be deposited forthe laser anode electrode pad. The active layer 720, sometimes referredto as an active region or an active area, may comprise waveguide layersin a cavity for generating optical signals.

Heater layers reside atop the laser layers and comprise, from bottom totop, a first Ti layer 760, an insulator layer 770 made of SiO₂, a secondTi layer 780, and a Pt layer 790. The second Ti layer 780 and the Ptlayer 790 together may serve as a heating pad for the heater since bothlayers are conductive and connected. During fabrication, to integratethe heater on the top of the Au sub-layer of the electrode layer 750,about 5 to 10 nm of Ti layer 760 may be deposited before about 200 nm ofthe insulator layer 770 is deposited. The two Ti layers 760 and 780 mayhelp bond SiO₂ in the insulator layer 770 to other metals. The insulatorlayer 770 may not be too thick so that it can allow efficient heattransfer from the electric heater to the burst-mode laser, and may notbe too thin so that it can block current injection from the heating padto the electrode pad. For example, the insulator layer 770 may have athickness between 50-200 nm, 100-300 nm, or 150-250 nm, or in othersuitable ranges.

FIG. 7B shows a perspective cross-sectional view of the laser system 600of FIG. 6 along the B-B line from a direction perpendicular to that ofFIG. 7A. The laser may be a ridge waveguide DFB laser with a heatersituated on top. According to the embodiment shown in FIG. 7B, theactive layer 720 is about 300 nm thick and may be on top of thesubstrate 710; the cladding layer 730 is about 1.5 μm thick; the ohmiccontact layer 740 is about 100 nm thick; the electrode layer 750 isabout 30 nm, 50 nm, and 500 nm thick for the respective layers of Ti,Pt, and Au; the first Ti layer 760 is about 5 nm thick; the insulatorlayer 770 may be about 200 nm thick; and the second Ti layer 780 and thePt layer 790 serving together as the heating electrode are 100 nm and300 nm, respectively. Further, the active layer 720 comprising a ridgewaveguide may have a width between about 2.5 μm to about 4 μm. Twotrenches 782 and 784, running parallel to the heating pad in the center,may each have a width between about 10 μm to about 15 μm. In the trencharea, an upper SiO₂ layer 786 may be about 200 nm thick, and a lowerSiO₂ layer 788 may be about 300 nm thick.

FIG. 7C shows a perspective cross-sectional view of a buriedheterostructure DFB laser, which has a heating pad 796 situated on topof an active region 791. For purposes of the present disclosure, theheater design and fabrication for FIG. 7C is largely similar to FIG. 7B,thus similarities are not further discussed in the interest ofconciseness. Distinct from a ridge waveguide laser, the buriedheterostructure DFB laser has an active region 791 fully embedded or“buried” between multiple n-InP layers 792 and p-InP layers 793.Further, a mesa width between the two trenches 794 and 795 may bebetween about 8 μm to about 12 μm, and the active region 791 may have awidth of about 1 μm to about 2 μm.

FIGS. 8A-8D illustrate embodiments of driving circuits 800, 830, 860,and 890, each for generating a heating current that feeds into a heaterdisclosed herein. Specifically, as shown in FIG. 8A, the driving circuit800 serves as a current source for a heater (e.g., the heater 620 inFIG. 6). The circuit 800 may comprise three parts: (1) a heater 802,which may be considered part of a DFB laser; (2) a constant currentsource 810 formed with a transistor 812 (denoted as Q2-1), a diode 814(denoted as LED2-1) and a resistor 816 (denoted as R2-1); and (3) acurrent modulator 820 formed with a control input voltage (Vctrl) 822, atransistor (Q2-2) 824 and a resistor (R2-2) 826. Exemplary values forsome of the components are given in FIG. 8A.

In the constant current source 810, the forward voltages of LED2-1 814may be 1.5 volts (V), and the emitter-base voltage (Veb) of Q2-1 812 maybe 0.6 V. Further, both voltages may vary with temperature following asimilar voltage-temperature curve. As a result, a difference between thetwo voltages, which may be applied onto R2-1 816, may be insensitive to(or substantially independent of) the ambient temperature. Consequently,R2-1 816 may be able to produce a constant current substantiallyindependent of the ambient temperature of the driving circuit. In otherwords, the current may be relatively stable when the temperaturechanges. Further, the value of R2-1 816 may be selected such that thecurrent is sufficient for heating the laser as discussed above.

In the current modulator 820, if Vctrl 822 is set to a low value orzero, no current may flow through Q2-2 824. In this case, the heater 802may accept all the current provided by the constant current source 810.Otherwise, If Vctrl 822 is set higher (e.g., higher than the turn-onthreshold voltage for Q2-2 824), at least part of the current from theconstant current source 810 may bypass through Q2-2 824 and R2-2 826,thereby reducing a current going through the heater 802. Thus, currentmodulation may be achieved by controlling a value of Vctrl 822. Theresponse time of the current modulator 820 may be designed to be short(e.g., about 1 nanosecond (ns)) to ensure fast switching or modulation.

One of ordinary skill in the art would recognize that the drivingcircuits 830, 860, and 890 shown in FIGS. 8B-8D are largely similar tothe driving circuit 800 in FIG. 8A. Thus, similarities are not furtherdiscussed. The driving circuit 830 in FIG. 8B may receive the output ofa timing circuit 832, which may control the timing of each stage in acurrent profile (e.g., how long a current stays at a maximal level, amedium level, and zero). The output current of the driving circuit 830can be preset to a certain value by adjusting the value of apotentiometer 834 (denoted as RV3). The driving circuit 830 may work atan on/off mode. The rising/falling time of the output current (e.g., thetime of reaching from the zero level to the maximal level) may be nomore than a few ns (e.g., about 1 ns).

The driving circuit 860 in FIG. 8C and the driving circuit 890 in FIG.8D may each be precisely controlled with a digital-to-analog (DA)converter, which may be 12 or 16 bits in depth. The outputs of the DAconverter in driving circuits 860 and 890 are labeled DA1 and DA2,respectively. During switching or modulation, the rising/falling time ofthe currents is about 1 μs. The main differences between drivingcircuits 860 and 890 are resistors values. In driving circuit 860,resistor 862 (R1-1) may be about 400 ohms and resistor 864 (R1-2) may beabout 1000 ohms. The driving circuit 860 may generate a relatively smalloutput current between about 0-2.5 mA, which may be suitable for a phasecontrol laser 866. On the other hand, in driving circuit 890, resistor892 (R2-1) may be about 40 ohms and resistor 894 (R2-2) may be about 100ohms. The driving circuit 890 may generate a relatively big outputcurrent between about 0-20 mA, which may be suitable for a DBR laser896.

FIG. 9 is a flowchart of an embodiment of a method 900 for temperaturecompensation during operation of a burst-mode laser. In a laser system(e.g., the laser system 600), the burst-mode laser may be a DFB laserthat is thermally coupled to a heater, which in turn is coupled to adriving circuit. The method 900 starts in step 910, when the drivingcircuit supplies an electric current to the heater for generating heatto be applied to the burst-mode laser. In an embodiment, the electriccurrent reaches a maximal level no later than the start of the emissionof the optical signal and then decreases from the maximal levelthereafter. In step 920, the burst-mode laser may receive a burst enablesignal indicating the start of a burst period. It should be understoodthat steps 910 and 920 may be implemented in any sequential order. Forexample, the electric current may be supplied to the heater eitherbefore the start of the burst period or simultaneously with the start ofthe burst period.

In step 930, the burst-mode laser may emit an optical signal with atleast one wavelength during the burst period. As the laser takes sometime to warm up, the emission of the optical signal may start shortly(e.g., about 1 μs) after the burst period. In step 940, the laser systemmay substantially maintain a temperature of the burst-mode laserthroughout the emission of the optical signal to reduce wavelength shiftof the optical signal. In the present disclosure, substantiallymaintaining or stabilizing a temperature may mean controlling thedrifting range within ±0.2 degree Celsius. Substantially maintaining thetemperature may be realized by applying, using the heater, heat to theburst-mode laser dynamically based on the burst enable signal. The heatmay be applied only during a first portion of the burst period and notduring a latter portion of the burst period.

FIG. 10 is a schematic diagram of an embodiment of a PON 100. The PON100 comprises an OLT 110, a set of ONUs 120, and an ODN 130 that may becoupled to the OLT 110 and the ONUs 120. The laser systems disclosedherein may be implemented in various components in the PON 100, such asin transmitters of the ONUs 120 for their upstream communications withthe OLT 110. The PON 100 may be a communications network that does notrequire any active components to distribute data between the OLT 110 andthe ONUs 120. Instead, the PON 100 may use the passive opticalcomponents in the ODN 130 to distribute data between the OLT 110 and theONUs 120. In an embodiment, the PON 100 may be a Gigabit PON (GPON),Next Generation Access (NGA) system, an Ethernet PON (EPON), a 10Gigabit EPON, a wavelength division multiplexing (WDM) PON, a TWDM PON,or other types of PON, or combinations thereof.

In an embodiment, the OLT 110 may be any device that is configured tocommunicate with the ONUs 120 and another network (not shown).Specifically, the OLT 110 may act as an intermediary between the othernetwork and the ONUs 120. For instance, the OLT 110 may forward datareceived from the network to the ONUs 120, and forward data receivedfrom the ONUs 120 onto the other network. Although the specificconfiguration of the OLT 110 may vary depending on the type of PON 100,in an embodiment, the OLT 110 may comprise a transmitter and a receiver.When the other network is using a network protocol, such as Ethernet orSynchronous Optical Networking/Synchronous Digital Hierarchy(SONET/SDH), that is different from the PON protocol used in the PON100, the OLT 110 may comprise a converter that converts the networkprotocol into the PON protocol. The OLT 110 converter may also convertthe PON protocol into the network protocol. The OLT 110 may be locatedat a central location, such as a central office, but may be located atother locations as well.

In an embodiment, the ODN 130 may be a data distribution system, whichmay comprise optical fiber cables, couplers, splitters, distributors,and/or other equipment. In an embodiment, the optical fiber cables,couplers, splitters, distributors, and/or other equipment may be passiveoptical components. Specifically, the optical fiber cables, couplers,splitters, distributors, and/or other equipment may be components thatdo not require any power to distribute data signals between the OLT 110and the ONUs 120. Alternatively, the ODN 130 may comprise one or aplurality of active components, such as optical amplifiers. The ODN 130may extend from the OLT 110 to the ONUs 120 in a branching configurationas shown in FIG. 10, but may be alternatively configured in any otherpoint-to-multi-point configuration.

In an embodiment, the ONUs 120 may be any devices that are configured tocommunicate with the OLT 110 and a customer or user (not shown).Specifically, the ONUs 120 may act as an intermediary between the OLT110 and the customer. For instance, the ONUs 120 may forward datareceived from the OLT 110 to the customer, and forward data receivedfrom the customer onto the OLT 110. Although the specific configurationof the ONUs 120 may vary depending on the type of PON 100, in anembodiment, the ONUs 120 may comprise an optical transmitter configuredto send optical signals to the OLT 110 and an optical receiverconfigured to receive optical signals from the OLT 110. Additionally,the ONUs 120 may comprise a converter that converts the optical signalinto electrical signals for the customer, such as signals in theEthernet or asynchronous transfer mode (ATM) protocol, and a secondtransmitter and/or receiver that may send and/or receive the electricalsignals to/from a customer device. In some embodiments, ONUs 120 andoptical network terminals (ONTs) are similar, and thus the terms areused interchangeably herein. The ONUs 120 may be typically located atdistributed locations, such as the customer premises, but may be locatedat other locations as well.

At least one example embodiment is disclosed and variations,combinations, and/or modifications of the example embodiment(s) and/orfeatures of the example embodiment(s) made by a person having ordinaryskill in the art are within the scope of the disclosure. Alternativeembodiments that result from combining, integrating, and/or omittingfeatures of the example embodiment(s) are also within the scope of thedisclosure. Where numerical ranges or limitations are expressly stated,such express ranges or limitations may be understood to includeiterative ranges or limitations of like magnitude falling within theexpressly stated ranges or limitations (e.g., from about 1 to about 10includes, 2, 3, 4, etc.; greater than 0.10 includes 0.11, 0.12, 0.13,etc.). For example, whenever a numerical range with a lower limit, R₁,and an upper limit, R_(u), is disclosed, any number falling within therange is specifically disclosed. In particular, the following numberswithin the range are specifically disclosed: R=R₁+k*(R_(u)−R₁), whereink is a variable ranging from 1 percent to 100 percent with a 1 percentincrement, i.e., k is 1 percent, 2 percent, 3 percent, 4 percent, 5percent, . . . , 50 percent, 51 percent, 52 percent, . . . , 95 percent,96 percent, 97 percent, 98 percent, 99 percent, or 100 percent.Moreover, any numerical range defined by two R numbers as defined in theabove is also specifically disclosed. The use of the term “about”means+/−10% of the subsequent number, unless otherwise stated. Use ofthe term “optionally” with respect to any element of a claim means thatthe element is required, or alternatively, the element is not required,both alternatives being within the scope of the claim. Use of broaderterms such as comprises, includes, and having may be understood toprovide support for narrower terms such as consisting of, consistingessentially of, and comprised substantially of. Accordingly, the scopeof protection is not limited by the description set out above but isdefined by the claims that follow, that scope including all equivalentsof the subject matter of the claims. Each and every claim isincorporated as further disclosure into the specification and the claimsare example embodiment(s) of the present disclosure. The discussion of areference in the disclosure is not an admission that it is prior art,especially any reference that has a publication date after the prioritydate of this application. The disclosure of all patents, patentapplications, and publications cited in the disclosure are herebyincorporated by reference, to the extent that they provide exemplary,procedural, or other details supplementary to the disclosure.

While several example embodiments have been provided in the presentdisclosure, it may be understood that the disclosed systems and methodsmight be embodied in many other specific forms without departing fromthe spirit or scope of the present disclosure. The present examples areto be considered as illustrative and not restrictive, and the intentionis not to be limited to the details given herein. For example, thevarious elements or components may be combined or integrated in anothersystem or certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various example embodiments as discrete or separatemay be combined or integrated with other systems, modules, techniques,or methods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and may be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. An apparatus comprising: a laser having anelectrode pad and configured to emit an optical signal during a burstperiod; a heater at least partially disposed along both sidewalls of theelectrode pad, the heater thermally coupled to the laser and configuredto apply heat to the laser; and a driver of an electric current forgenerating the heat, wherein the heat is applied to the laser before astart of the burst period to substantially stabilize a temperature ofthe laser during the burst period.
 2. The apparatus of claim 1, whereinthe electric current is dedicated to the heater.
 3. The apparatus ofclaim 1, wherein a duration of the heater applying the heat is about 125microseconds (μs), wherein the temperature varies no more than about 0.2degrees Celsius (° C.) when the heater applies the heat, and wherein awavelength shift is no more than about 0.02 nanometers (nm) when theheater applies the heat.
 4. The apparatus of claim 1, further comprisinga driving circuit coupled to the heater and comprising a constantcurrent source configured to generate the electric current.
 5. Theapparatus of claim 4, wherein the constant current source comprises: atransistor comprising a base; and a diode coupled to the base, andwherein the transistor and the diode have similar voltage-temperaturecharacteristics such that the transistor and the diode are configured tocreate a constant voltage difference that is substantially independentof an ambient temperature of the driving circuit.
 6. The apparatus ofclaim 1, wherein the laser is a distributed feedback (DFB) laser,wherein the heater comprises a silicon dioxide (SiO₂) layer, and whereina thickness of the SiO₂ layer allows heat transfer from the heater tothe laser, but blocks current injection from the heater to the laser. 7.An apparatus comprising: a laser having an electrode pad and configuredto emit an optical signal during a burst period; a heater integrated atleast partially over a top surface and along both sidewalls of theelectrode pad, the heater thermally coupled to the laser and configuredto: receive an electric current at or before a start of the burstperiod, wherein the electric current is used by the heater to generateheat; and apply the heat to the laser at the start of the burst periodto stabilize a temperature of the laser during the burst period.
 8. Theapparatus of claim 7, wherein the electric current: reaches a firstlevel from zero in no more than about 2 nanoseconds (ns); stays at thefirst level for no more than about 1 microsecond (μs); decreases to asecond level; stays at the second level for no more than about 10 μs;and decreases to zero.
 9. The apparatus of claim 8, wherein the electriccurrent continuously decreases from the first level to zero by followingan exponential curve.
 10. A method for temperature compensation duringoperation of a laser that is thermally coupled to a heater, the methodcomprising: receiving an enable signal; emitting an optical signal withat least one wavelength during a burst period; supplying an electriccurrent to the heater to generate heat, the electric current havingreached a maximal level at least about 4 microseconds (μs) before theburst period and decreases to a second level until an application of theheat to the laser is terminated; applying the heat to the laser before astart of the burst period based on the enable signal; substantiallymaintaining a temperature of the laser throughout an emission of theoptical signal; and terminating the application of the heat to the laserat the end of the burst period.
 11. The method of claim 10, wherein aduration of the application of heat is about 125 microseconds (μs),wherein the temperature of the laser varies no more than about 0.2degrees Celsius (° C.) during the application of heat, and wherein awavelength shift is no more than about 0.02 nanometers (nm) during theapplication of heat.
 12. The method of claim 10, further comprisingsimultaneously receiving the electric current and the enable signal atthe start.
 13. The method of claim 10, wherein the electric current:stays at the maximal level for no more than about 1 μs; and stays at thesecond level for no more than about 10 μs.
 14. The method of claim 10,further comprising supplying the electric current to the heater using adriving circuit, wherein the driving circuit comprises a constantcurrent source, wherein the constant current source comprises atransistor and a diode coupled to the transistor, and wherein thetransistor and the diode have similar voltage-temperaturecharacteristics.
 15. The method of claim 14, further comprisingcreating, using the transistor and the diode, a constant voltagedifference that is substantially independent of an ambient temperatureof the driving circuit.
 16. A laser system comprising: a lasercomprising a metallic layer configured to serve as an electrode pad forthe laser; and an electric heater situated atop the laser andcomprising: a first titanium (Ti) layer atop the metallic layer; asilicon dioxide (SiO₂) layer atop the first Ti layer, wherein the SiO₂layer is configured to allow efficient heat transfer from the electricheater to the laser; a second Ti layer positioned atop the SiO₂ layer;and a platinum (Pt) layer atop the second Ti layer, wherein the secondTi layer and the Pt layer are configured to serve as a pad for theelectric heater.
 17. The laser system of claim 16, wherein the SiO₂layer comprises a first thickness of no less than about 100 nm, whereinthe first Ti layer comprises a second thickness of about 4 nm to about10 nm, and wherein the second Ti layer has a third thickness of about280 nm to about 320 nm.
 18. The laser system of claim 16, wherein thelaser further comprises: an active layer configured to generate anoptical wave when an enable voltage is applied on an electrode pad; acladding layer atop the active layer and comprising p-type indiumphosphide (p-InP); and an ohmic contact layer atop the cladding layer,underneath the metallic layer, and comprising heavily-doped p-typeindium gallium arsenide (p+-InGaAs), wherein the electric heater isconfigured to apply heat to the active layer based on the enable voltagesuch that a temperature of the active layer is substantially stabilizedduring a period.
 19. The laser system of claim 16, further comprising agold (Au) layer atop a portion of the Pt layer, wherein the Au layer isconfigured to serve as a bonding pad.
 20. An apparatus comprising: alaser having an anode and configured to emit an optical signal during aburst period; and a heater at least partially disposed over a topsurface and both sidewalls of the anode, the heater thermally coupled tothe laser and configured to apply heat to the laser, wherein the heat isapplied to the laser at or before a start of the burst period and thenreduced during the burst period to substantially stabilize a temperatureof the laser during the burst period.
 21. The apparatus of claim 20,further comprising a driver circuit configured to supply an electriccurrent to the heater, wherein the electric current causes the heater togenerate the heat applied to the laser.
 22. The apparatus of claim 20,wherein application of the heat to the laser at or before the burstperiod mitigates wavelength shift of the optical signal during the burstperiod.
 23. The apparatus of claim 20, wherein an amount of the heatapplied to the laser is reduced after the start of the burst period. 24.The apparatus of claim 20, wherein the heater is turned off before theburst period has ended.
 25. The apparatus of claim 20, wherein the laseris disposed within a transmitter of an optical network unit (ONU). 26.The apparatus of claim 20, wherein the laser is disposed within atransmitter of an optical line termination (OLT).